What is legal discrimination?
This depends on how you mean this term. There are two meanings of this term that I am familiar with.
First, there is the idea of de jure discrimination -- that is discrimination that is imposed by law. An example of this was the Jim Crow system that was in effect in much of the United States in the first half of the 20th century. This is in contrast to discrimination that individual people engage in (like if you, yourself, will not allow someone of a different race or religion to enter your home).
Second, there are kinds of discrmination that are legal. The Halloran-Sage link discusses the idea that it is legal (in the US) to refuse to hire some kind of person as long as there is a legitimate reason for it. For example, it would be legal to say that people over a certain age can't be airline pilots because you don't want a pilot dying while trying to land a plan full of passengers.